1. Field of the Invention
The present invention relates to a multi-antenna system which is applicable to a system requiring multiple antennas, and more particularly, to a multi-antenna system using adaptive beamforming, which is capable of rapidly performing beamforming for multiple antennas.
2. Description of the Related Art
In general, a wireless communication system using multiple antennas or spaced antennas (hereinafter, referred to as “multi-antennas”) for high-speed wireless communications is being developed. Beamforming is one of several technologies using multi-antennas and is widely known as a method in which a receiver or transmitter uses multi-antennas to increase connection reliability in wireless environments.
Worldwide interoperability for microwave access (WiMax) standard, long-term evolution (LTE), IEEE802.11n WLAN, IEEE802.15.c WPAN and so on may be taken as examples of wireless environments in which such multi-antennas are used. In order to implement a multi-antenna system, an equal number of radio frequency (RF) chain units configured with a low noise amplifier (LNA), a mixer, a filter, an intermediate frequency (IF) signal, and an analog-to-digital (A/D) converter are needed. Therefore, the price, power consumption, and size of a multi-antenna system are being considered as problems in implementing the multi-antenna system.
In particular, it is known that an A/D converter has the highest power consumption when processing baseband signals. To minimize this power consumption, an analog beamforming or RF beamforming technology employing a minimum number of RF components may be used. In the existing baseband beamforming technology, signals received by antennas should be converted into digital signals through an A/D converter so as to calculate a weight vector which maximizes a signal-to-noise ratio (SNR).
However, when the analog beamforming technology is used, the phases of signals received by antennas are converted by phase shifters and then summed. Therefore, only one A/D converter having high power consumption may be used. Accordingly, research and development on the analog beamforming technology has been conducted intensively.
In the baseband beamforming technology according to the related art, signals received by multi-antennas may be converted into digital signals through an A/D converter so as to calculate an optimal weight vector through eigenvalue decomposition. When this technology is used, the direction of a signal received by an antenna may be accurately estimated. Therefore, it is possible to improve the performance of the multi-antenna system. However, since the number of RF chain units increases in proportion to the number of antennas, the cost of the multi-antenna system increases by the same amount, and the power consumption of the multi-antenna system increases.
To overcome such disadvantages, analog beamforming technology has been used. In the analog beamforming technology, the phases of signals received by multi-antennas are properly shifted by the phase shifters, instead of passing through RF chains, and the phase-shifted signals are then combined together.
In a technique which estimates a weight vector using the analog beamforming technology according to the related art, all vectors are applied to find an optimal vector.
However, since finding an optimal vector for all vectors may increase the complexity of a system, it is difficult to apply to a real system. In another technique, an orthogonal matrix including the entire vector space may be preset and shared by a receiver and a transmitter, and an optimal vector may be searched for within the matrix through repeated training signals. In this technique, it takes a lot of time to find the optimal vector because all vectors should be searched to find an optimal vector.